Three-dimensional MHD simulation of an emerging flux tube in the sun

Abstract

We have done 3-dimensional MHD simulations of a magnetic flux tube emerging through the solar atmosphere and studied not only the dynamics but also the geometric structure of emerging magnetic field. A flux tube twisted in the left-handed sense (Gold-Hoyle flux tube) is initially placed below the photosphere, and thereafter starts to rise with an upward motion induced to the middle part of the flux tube. When the outer edge of the flux tube crosses the photosphere, it is found that emerging field lines are almost perpendicular to the neutral line and the gas flow induced by the emergence diverges from the emerging region. As emergence proceeds, the direction of emerging field lines rotates to align with the neutral line and the flow direction changes from a diverging pattern (perpendicular to the neutral line) into a shearing pattern (parallel to the neutral line). As for the dynamical behavior of emerging field lines, it is found that there are two kinds of evolutionary phases: an expansion phase and a gradual phase. Outer field lines of the flux tube, which emerge earlier than inner field lines, simply expand when they enter the solar atmosphere. On the other hand, inner field lines show a gradual phase in which the field lines rise slowly with a waving motion. We have also investigated the structure of emerging field lines. Simulations show that outer field lines form an arcade over the neutral line that resembles a potential field. On the other hand, those inner field lines that are initially located just beneath the tube axis show an inverse-S structure. Observationally this structure is known as a sigmoid, an assembly of bright soft X-ray coronal loops, with an S or an inverse-S shape. By showing how current density is distributed around the footpoints of individual field lines, we provide a possible explanation for why a certain group of emerging field lines are illuminated to form a sigmoid.